1. Field of the Invention
The present invention relates to a laser speckle velocity-measuring apparatus in which a laser beam is irradiated onto a moving object. Information represented by a speckle pattern which is caused by the scattered light from the moving object is detected, and the velocity of the moving object is measured.
2. Description of the Prior Art
Various kinds of methods have been proposed to measure the velocity by utilizing a laser speckle. A cross-correlation method is known as one of those methods. The principle of the cross-correlation method is shown in FIG. 1. A laser beam LR is irradiated onto an object OB which moves at a velocity V. Although the reflected light from the object OB is scattered and spreads into the space, this scattered light produces a clear dark and light speckle pattern due to the coherent property of the laser beam LR. The speckle pattern moves in conjunction with the movement of the object OB. This moving speckle pattern is detected at first and second photo sensing positions 1 and 2 which are arranged at an interval X.sub.d along the moving direction of the object OB. Reference numerals 1 and 2 indicate the light receiving positions of the photo sensing devices and the photo sensing devices do not need to be arranged at these positions.
Each of the photo sensing devices 1 and 2 converts the received optical signal to the analog electric signal. This electric signal is binarized by a comparator and becomes a binary signal. The binary signal obtained by the second photo sensing device 2 (this signal is referred to as the "delay signal" hereinafter) is detected with a certain delay time .tau..sub.d from the binary signal derived from the first photo sensing device 1 (this signal is referred to as the "preceding signal" hereinbelow).
As shown in FIG. 2, n.sub.1 bits of the preceding signal are stored into a memory 5 and n.sub.2 bits (n.sub.2 &gt;n.sub.1) of the delay signal are stored into a memory 6 each bit at a predetermined sampling period T, respectively. Thereafter, each bit data of the preceding signal is compared with each predetermined bit data of the delay signal by a coincidence deciding circuit 7, thereby discriminating whether the contents of these data coincide or not.
In this case, a check is first made to see if the data of the first to (n.sub.1)th bits of the preceding signal coincides with the data of the first to (n.sub.1)th bits of the delay signal or not. Thus, the number of bits whose data contents coincide is counted by a counter 8, thereby obtaining the coincidence degree (correlation degree: correlation value) X. The value X denotes the correlation value when the time shift of the memory 6 is set to zero (in the diagram, this state is shown by K=0).
Next, a check is made to see if the data of the first to (n.sub.1)th bits in the memory 5 coincides with the data of the second to (n.sub.1 +1)th bits in the memory 6 or not. The correlation value, that is, the coincidence degree X in a case where the time shift of the memory 6 is set to T (K=1) is obtained by the counting operation of the counter 8.
In a manner similar to the above, the correlation values when the time shift is set to T.times.j (where, j=0, 1, 2, . . . ) are obtained, so that a correlation function curve CC having a correlation peak P is obtained as shown in FIG. 3. The delay time .tau..sub.d until the correlation peak P is obtained by the correlation function curve CC. By substituting this delay time .tau..sub.d in the following equation (1), the velocity v of the object OB is calculated. Further, by integrating the velocity v by the time, the length or displacement (moving distance) of the object OB can be calculated. ##EQU1## where, X.sub.d denotes the distance between the photo sensing devices 1 and 2 (between the light receiving points) and k.sub.c is a constant.
In the case of the foregoing system, after the necessary data regarding the preceding and delay signals were stored into the memories 5 and 6, the data input is inhibited and the correlating process is executed by driving the memories. Therefore, it takes a long time to detect the correlation peak and, further, to measure the length or velocity of the object. In particular, there is a problem such that it is difficult to perform the high speed processes in a real-time manner.